Many power generation plants produce electricity by converting various forms of energy (e.g. fossil fuel, nuclear fuel, hydro or wind flow, and geothermal heat) into mechanical energy (e.g. rotation of a turbine shaft), and then converting the mechanical energy into electrical energy (e.g. by the principles of electromagnetic induction).
Some of these power generation plants, such as a fossil-fuel power generation plant, may comprise a turbine, a generator and an exciter. The turbine, generator and exciter are typically coupled to each other in axial alignment, with the generator located between the turbine and the exciter.
The turbine converts fossil fuel energy into mechanical energy in the form of turbine shaft rotation through a steam or combustion cycle. The generator then converts the rotational energy into electrical energy. The generator includes an axially extending rotor journaled in an annular stator that surrounds the rotor. The rotor has a shaft in which conductive coil windings may be axially arranged. The stator has punchings that collectively form an annular core in which conductive coil windings are positioned generally parallel with respect to the axial rotor coils. As the turbine shaft rotates the generator rotor, the exciter provides an electrical current to the rotor coil windings. The rotating electrically excited rotor creates a magnetic flux that induces an electrical current in the stationary stator coil windings. This induced electrical current constitutes the electricity that the power generation plant supplies to consumers of electricity.
One aspect of the foregoing power generation operation involves the electrical interconnection of the exciter and generator. An electrically conductive lead path is used to carry current in a closed loop configuration from the exciter, through the generator rotor coil windings, and then back to the exciter.
It is known that repeated start/stop cycling for generators of large size and weight creates substantial inertial and thermal forces that induce mechanical stresses on the various components of such generators. Components situated at some radial distance from the rotor axis may be subjected to significant centrifugal forces. Such components may include field coils disposed about the rotor and restrained from moving outward away relative to the rotor axis by restraining structures, such as adhesives, coil wedges, retaining rings and other restraining devices.
Electrically connecting structures, traditionally referred to in the art as a J-strap, that connect the field coils to terminals for establishing an electrically conductive lead path to the exciter, for example, may be subjected to the above-described forces, including forces tending to axially and/or radially displace the field coils relative to the rotor each time the generator is started or stopped. Concern has arisen that such connecting structures may become potential weak links in such generators. Lead path failure can cause electric arcing or re-routing of the electric current through nearby conductive materials. Arcing and re-routing can melt portions of the rotor shaft and otherwise damage the generator.